ヒトの祖型の生体力学的探求 : 直立2足歩行の起源

抄録

From what sort of animal and in what process did humans obtain bipedal locomotion through evolution? This is the most fundamental question still left to us for understanding the origin of hominid bipedalism. Recently, in considering prehabitual locomotion, the brachiation model and vertical climbing model have become leading theories. However, these proposed models do not explain why the body proportions of man and modeled living brachiaters or climbers are so different in terms of the intermembral index. In this study, we analyzed several types of human bipedal and quadrupedal walking with different intermembral indices. The intermembral index was changed using a prosthesis (1 kg) which can elongate the arm from 70% to 140% in 10% steps. Bipedal walking of an ape, which is characterized by an inclined body and flexed knee joint, was imitated by human subjects. In quadrupedal walking, the knee joint was rather extended like that of a monkey. We also analyzed climbing motion on a ladder as a substitute for vertical climbing. The height of the experimental ladder was 4.2m, and the width 80cm. The distance between each step is 10cm, so that a subject could freely place a foot on a nearby step. To detect vertical and horizontal forces on feet and hands, force sensors were attached to 10 steps partially up the ladder. These walking and climbing motions were measured three-dimensionally by two sets of position detector cameras. The musculo-skeletal model was composed of 7 rigid segments-thigh, shank, foot, upper arm, forearm, and head-torso segments-and 9 principal muscles in each lower limbs. Model parameters such as mass were adjusted to each experimental subject. Some evaluative indices for the joint motions and muscle activities were defined by magnitude and pattern, so that we can quantitatively compare the similarity to erect bipedal walking. Calculating the distance of the indices from erect bipedal walking, we obtained the following results: 1) Quadrupedal walking enhances the development of knee muscles. 2) Vertical climbing improves ankle extensors and the kinematic potentiality of each joints. 3) Ape-type bipedal walking strengthens the muscles around the hip and ankle joints. 4) The intermembral index, which changes from quadrupedal walking to ape-type bipedal walking, and subsequently from ape-type bipedal walking, to erect bipedal walking is about 110 or 140. Therefore, we conclude that the change toward bipedal walking occurred in primates with long upper extremities, and that vertical climbing assists the change from quadrupedal to bipedal walking mechanically as well as kinesiologically. This conclusion, which proposes that primates with longer upper extremities were more capable of changing toward bipedalism, does not contradict the brachiation model. Moreover, those who keep their bodies away from trees (ladders) have more potential ability for bipedalism; this also shows the adaptability of primates with long upper extremities as our ancestors.